Rubber reinforcing steel cord and heavy duty pneumatic radial tire using the steel cord

ABSTRACT

A rubber reinforcing steel cord of a bundle-twisted+layer-twisted structure obtained by arranging 9 steel filaments around 3 steel filaments each being two-dimensionally wave-like shaped in the lengthwise direction, intertwining all of these steel filaments in one operation to form a bundle-twisted cord, and further arranging 14 steel filaments around the bundle-twisted cord followed by intertwining. Also, a heavy duty pneumatic radial tire using the rubber reinforcing steel cord as the cord of the carcass layer thereof.

TECHNICAL FIELD

The present invention relates to a rubber reinforcing steel cord, whichis excellent in the rubber permeability and the fatigue resistance andalso has a high productivity, and to a heavy duty pneumatic radial tireusing the steel cord as the cord for the carcass layer.

BACKGROUND OF THE INVENTION

Hitherto, in a heavy duty pneumatic radial tire used for large vehiclessuch as trucks, buses, etc., in order to make endurable heavy load,steel cords are frequently used as cords for the carcass layers formingthe tire skeleton.

As the steel cord, as shown in FIG. 3, a layer-twisted structure of3+9+15+W obtained by intertwining nine (9) steel filaments 2 around acore portion formed by intertwining three (3) steel filaments 1,intertwining 15 steel filaments 3 around the 9 steel filaments 2, andfurther binding them by wrapping a wrapping filament W around theoutside of them; and a bunched structure (bundle-twisted structure) of1×12+W obtained by intertwining 12 steel filaments and binding them bywrapping a wrap filament around the outside of them, or of 1×27+Wobtained by intertwining 27 steel filaments and binding them by wrappinga wrap filament around the outside of them are generally used. Inaddition, the wrap filament is for binding the intertwined steelfilaments so that the cord is not dispersed.

However, in these steel cords of the layer-twisted structure or thebunched structure, there was a problem that because of the existence ofthe wrap filament, fretting occurred between the wrap filament and thesteel filaments just under the wrap filament during the use to lower thestrength of the steel filament, which resulted in lowering the fatigueresistance of the steel cord.

Also, in the steel cord of the layer-twisted structure, there was aproblem that because of the closest structure having no gaps amount thesteel filaments, a rubber permeability into the inside of the cord wasbad, whereby when water penetrated during the use or storage of the tirereached the cord, the cord was corroded to lower the corrosionresistance and the fatigue resistance. On the other hand, in the steelcord of the bunched structure, the productivity can be improved becausethe number of the production steps is less as compared with the steelcord of the layer-twisted structure but there was a problem that becausethe structure was also the closest structure as the layer-twistedstructure, the rubber permeability into the inside of the cord was bad.

Thus, recently, for improving lowering of the fatigue resistance byfretting, a steel cord having no rap filament is proposed (Japanesepatent application Kokai publication No. 8-176977), wherein the cord isa three-layer structure as shown in FIG. 4 but the number of steelfilaments of out outermost layer is reduced by one or two than theclosest number (the maximum number capable of being disposed) of steelfilaments to increase the rubber permeability of the outermost layer(3+9+14 layer-twisted structure in FIG. 4) and the cord is bound by thepermeated rubber. However, in the steel cord, because other insidelayers than the outermost layer are the closest structure, the rubberpermeability is insufficient.

Also, a steel cord of the layer-twisted structure or the bunchedstructure using at least one wave-like shaped steel filament at the coreportion for increasing the rubber permeability is proposed (Japanesepatent application Kokai publication No. 9-31875). However, in the steelcord, the rubber permeability is improved but the form of the crosssection of the cord does not become a homogeneous round (becomesdistorted), when the steel cord is used as a cord of the carcass layerand a largely bending input force is applied to the cord, an ununiformstrain occurs in the cord, whereby the fatigue resistance of the cord islowered. An embodiment of the cord is shown in FIG. 5. The embodiment ofFIG. 5 is a structure that one of three steel filaments of the coreportion is wave-like shaped and in the embodiment, a steel cord of alayer-twisted structure of 3Cr+9+14 is constructed by intertwining 9steel filaments 2 around the core portion formed by intertwining 3 steelfilaments including the one above-described steel filament andintertwining 14 steel filaments 3 around the 9 steel filaments 2(wherein, Cr means a wave-like shaped form). However, in the steel cordof FIG. 5, the cord is produced by three times twisting, which isdisadvantageous in the production cost.

As other steel cords, for example, the steel cords shown in FIG. 6 andFIG. 7 are proposed. In the embodiment of FIG. 6, a steel cord of thebunched structure of 3Cr/24 obtained by disposing total 24 steelfilaments 2 and 3 around three(3) steel filaments 1 wave-like shaped inthe lengthwise direction is constructed. In the steel cord of FIG. 6,the productivity is high because the production step is one-timetwisting only but because the cross-sectional form of the cord is nearlya triangle (straw bag form) and thus a strain becomes ununiform, thefatigue resistance is inferior and also because the steel cord hasalmost the closest structure, the rubber permeability is insufficient.In the embodiment shown in FIG. 7, a steel cord of the layer- twistedstructure of 3Cr+9+14 obtained by intertwining 9 steel filaments 2around a core portion formed by arranging 3 steel filaments 1 wave-likeshaped in the lengthwise direction without intertwining, andintertwining 14 steel filaments 3 around the 9 steel filaments 2 isconstructed. In the steel cord of FIG. 7, because 3Cr of the coreportion are straight, when a cord tension is applied, the tension burdenis largely applied to the core portion (the core portion is stretched),the partial charge of the tension becomes ununiform and the fatigueresistance is no good.

DISCLOSURE OF THE INVENTION

The present invention provides a rubber reinforcing steel cord beingexcellent in the rubber permeability and the fatigue resistance and alsohaving a high productivity and also the invention provides a heavy dutypneumatic radial tire using the above-described steel cord as the cordof the carcass layer.

That is, the rubber reinforcing steel cord of the invention is a steelcord of a bundle-twisted+layer-twisted structure obtained by arranging 9steel filaments around three steel filaments 1 each beingtwo-dimensionally wave-like shaped in the lengthwise direction,intertwining all of these steel filaments in one operation to form abundle-twisted cord, and further arranging 14 steel filaments around thebundle-twisted cord followed by intertwining, wherein when the diameterof the two-dimensionally wave-like shaped steel filament is d, thewave-form height thereof is h, and the wave form pitch thereof is P, theparameter F shown by (h−d)/P is in the relation of 0.001≦F≦0.03 andfurther, the ratio b/a of the long diameter a′ and the short diameter bof the circumscribed circle of the cross section of the bundle-twistedcord is 0.94≦b/a≦1.00, and the ratio b′/a′ of the long diameter a′ andthe short diameter b′ of the circumscribed circle of the cross sectionof the steel cord is 0.96≦b′/a′≦1.00.

As described above, in the invention, because the 3 steel filaments ofthe core portion are wave-like shaped in the lengthwise direction, theexistence of gaps in the core portion is ensured and thus the rubberpermeability of the cord becomes good. Also, because the steel cord ofthe invention has the bundle-twisted+layer-twisted structure having theabove-described core portion, the cross sectional form of the steel cordcan be made almost homogeneous and round and thus, when a large bendinginput force is applied to the steel cord, an ununiform strain does notoccur in the steel cord, whereby the fatigue resistance of the steelcord is not lowered. Furthermore, because the steel cord of theinvention has the bundle-twisted+layer-twisted structure, the number ofthe production steps can be reduced as compared with the relates artlayer-twisted structure, whereby the productivity can be increased.

Also, in the heavy duty pneumatic radial tire of the invention using theabove-described rubber reinforcing steel cord excellent in the rubberpermeability and the fatigue resistance as described above as the cordof the carcass layer, the durability can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of the rubberreinforcing steel cord of the invention,

FIG. 2 is a side view explaining an embodiment of the steel filamentconstituting the core portion of the rubber reinforcing steel cord ofthe invention,

FIG. 3 is a cross-sectional view of an embodiment of a related artrubber reinforcing steel cord,

FIG. 4 is a cross-sectional view of other embodiment of a related artrubber reinforcing steel cord,

FIG. 5 is a cross-sectional view of still other embodiment of a relatedart rubber reinforcing steel cord,

FIG. 6 is a cross-sectional view of other embodiment of a related artrubber reinforcing steel cord,

FIG. 7 is a cross-sectional view of still other embodiment of a relatedart rubber reinforcing steel cord, and

FIG. 8 is a cross-sectional view of an embodiment of the rubberreinforcing steel cord wherein the outermost layer is constituted of 13steel filaments.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, the rubber reinforcing steel cord 11 of theinvention is the steel cord having a bundle-twisted+layer-twistedstructure (the twisted structure of 3 Cr/9+14) obtained by arranging 9steel filaments 2 around three steel filaments 1 each-beingtwo-dimensionally wave-like shaped in the lengthwise direction,intertwining all of these steel filaments in one operation to form abundle-twisted cord 10, and further arranging 14 steel filaments 3around the bundle-twisted cord 10 followed by intertwining.

Each of the three steel filaments of the core portion istwo-dimensionally wave-like shaped in the lengthwise direction as shownin FIG. 2, and when the diameter of the steel filament is d, the waveform height thereof is h, and the wave form pitch thereof is P, theparameter F shown by (h−d)/P is in the relation of 0.001≦F≦0.03.

When F is less than 0.001 (F<0.001), the shape of the wave form becomesnear straight, whereby gaps do not form amount the 9 element (steelfilaments) outside the core portion and thus the rubber permeabilitybecomes inferior. Also, when F exceeds 0.03 (F>0.03), when a cordtension is applied, the burden of the tension become ununiform betweenthe core portion and the side portion to lower the fatigue resistance.

In the invention, because the steel cord is thebundle-twisted+layer-twisted structure having the above-described coreportion, the cross sectional form of the steel cord can be make almosthomogeneous and round but in order to make more round, the ratio b/a ofthe long diameter a and the short diameter b of the circumscribed circleof the cross section of the bundle-twisted cord 10 is 0.94≦b/a≦1.00, andthe ratio b′/a′ of the long diameter a′ and the short diameter b′ of thecircumscribed circle of the cross section of the steel cord 11 is0.96≦b′/a′≦1.00. Thereby, when a large bending input force is applied tothe steel cord 11, the fatigue resistance of the steel cord can be moreincreased. Also, by making the cross sectional form such a round, thedispositions of the steel filaments 1, 2, and 3 become line symmetry,whereby the occurrence of an ununiform strain in the steel cord 11 canbe prevented.

The number of the steel filaments 3 constituting the outermost layer ofthe steel cord 11 is 14 which is less than the closest number (themaximum number capable of being disposed, that is 15) by one. By using14 steel filaments, gaps form between each filaments in the outermostlayer, whereby the penetration of a rubber into the inside of the steelcord becomes easy. Thereby, a wrapping filament can be omitted. Also,when the number of the steel filaments constituting the outermost layeris less than 14, for example, is 13 as shown in FIG. 8, the diameter ofthe steel filaments must be increased to keep the necessary strength asthe cord, whereby the fatigue resistance is undesirably lowered.

Also, from the view point of the production cost of the steel cord, itis preferred that the diameter of each of the steel filaments 1, 2, and3 constituting the steel cord 11 is same. Furthermore, form the pointsof the productivity and the form of the cord, it is preferred that thetwisting pitch P₁ of the bundle-twisted cord 10 and the twisted pitch P₂of the plural filaments 3 of the outermost layer disposed around thebundle-twisted cord 10 are in the relation of P₂≧2.5+P₁. This isbecause, if P₂<2.5+P₁, the filaments of the outermost layer are liableto be fallen in the bundle-twisted cord 10 and also the productivitybecomes bad.

EXAMPLES, RELATED ART EXAMPLES, AND COMPARATIVE EXAMPLES

Using the steel cords having the various constructions shown in Table 1and Table 2 below as the cords (carcass cords) of carcass layers, heavyduty pneumatic radial tires of a tire size of 1000R 20 14PR (Examples 1to 6, Related Art Examples 1 and 2, and Comparative Examples 1 to 8)were prepared and about these tires, the rubber permeability and thefatigue resistance were evaluated. The results are also shown in Table 1and Table 2 together. In addition, in Table 1 and Table 2, “Core 3inside” means the rubber permeability of the inside of the core of 3crimped steel filaments, “Between core 3 and side 9” means the rubberpermeability between 3 crimped steel filaments of the core and 9 steelfilaments of outside of the core, and “Between side 9 and outermostsheath” means the rubber permeability between the 9 steel filaments ofthe outside of the core and the steel filaments of the outermost sheathor layer.

TABLE 1 Related art Related art Example 1 Example 2 Example 3 Example 4Example 5 Example 6 example 1 example 2 Twisted 3Cr/9 + 14 3Cr/9 + 143Cr/9 + 14 3Cr/9 + 14 3Cr/9 + 14 3Cr/9 + 14 3 + 9 + 15 + w 3 + 9 + 14structure Cross-sectional FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG.3 FIG. 4 view of steel cord Filament 0.170 0.170 0.170 0.170 0.170 0.1700.170 0.170 diameter (mm) Crimped steel 3 3 3 3 3 3 0 0 filament numberRoundness b/a 0.945 0.945 0.945 0.985 0.94 0.945 0.98 0.96 b′/d′ 0.970.97 0.97 0.99 0.96 0.96 0.985 0.97 Parameter F 0.003 0.001 5 0.0250.003 0.003 0.003 — — Pitch P₁/P₂ 10/16 10/16 10/16 10/16 10/16 10/12 —— Rubber permeability (%)*¹ Core 3 inside 50 30 50 50 50 30 0 0 Betweencore 3 80 65 85 80 80 75 15 15 and side 9 Between side 9 95 90 95 95 9590 30 80 and outermost sheath Fatigue 95 90 90 97 88 88 30 60 retention(%) (fatigue resistance)*² Note) *¹The rubber permeability of a carcasscord taken out from a new tire. *²Each cord was taken out from a tireafter running 200,000 km and a new tire, a rotary bending fatigue test(under constant stress) was applied to each cord, the rotation numberuntil the cord was broken was measured. (value after running/value atnew × 100 (%))

TABLE 2 Comparative Comparative Comparative Comparative ComparativeComparative Comparative Comparative Example 1 Example 2 Example 3Example 4 Example 5 Example 6 example 7 example 8 Twisted 3Cr + 9 + 143Cr/24 3Cr + 9 + 14 3Cr/9 + 14 3Cr/9 + 14 3Cr/9 + 14 3Cr/9 + 14 3Cr/9 +13 structure Cross-sectional FIG. 5 FIG. 6 FIG. 7 FIG. 1 FIG. 1 FIG. 1FIG. 1 FIG. 8 view of steel cord Filament 0.170 0.170 0.170 0.170 0.1700.170 0.170 0.175 diameter (mm) Crimped steel 1 3 3 3 3 3 3 3 filamentnumber Roundness b/a 0.90 0.85 0.92 0.945 0.945 0.92 0.92 0.945 b′/d′0.96 0.85 0.94 0.97 0.97 0.97 0.95 0.97 Parameter F 0.049 0.049 0.0490.0007 0.035 0.003 0.003 0.003 Pitch P₁/P₂ — — — 10/16 10/16 10/16 10/16— Rubber permeability (%)*¹ Core 3 inside 0 0 0 0 50 50 50 50 Betweencore 3 40 20 35 30 80 80 80 80 and side 9 Between side 9 85 80 85 80 9595 95 100 and outermost sheath Fatigue 60 30 70 60 70 80 75 75 retention(%) (fatigue resistance)*² 2 Note) *1 and *2 are same as in Table 1.

In Table 1, Example 6 is the case of P₂<2.5+P₁, and because in Example6, falling of the filaments of the outermost layer into thebundle-twisted cord occur, the roundness and the rubber permeabilitybecome inferior to some extent. Also, in Table 1, Related art example 1is the case of using the steel cord of the layer-twisted structure of3+9+15+W as shown in FIG. 3 and Related art example 2 is the case ofusing the steel cord of the layer-twisted structure of 3+9+14 as shownin FIG. 4.

In Table 2, Comparative Example 1 is the case of using the steel cord ofthe layer-twisted structure of 3Cr+9+14 as shown in FIG. 5, ComparativeExample 2 is the case of using the steel cord of the layer-twistedstructure of 3Cr/24 as shown in FIG. 6, and Comparative Example 3 is thecase of using the steel cord of the layer-twisted structure of 3Cr+9+14as shown in FIG. 7. Also, Comparative Example 4 is the case that thesteel cord of the layer-twisted structure of 3Cr/9+14 as shown in FIG. 1is used but the parameter F is too small, Comparative Example 5 is thecase that the steel cord of the layer-twisted structure of 3Cr/9+14 asshown in FIG. 1 is used but the parameter F is too large, ComparativeExample 6 is the case that the steel cord of the layer-twisted structureof 3Cr/9+14 as shown in FIG. 1 is used but the roundness b/a is toosmall, Comparative Example 7 is the case that the steel cord of thelayer-twisted structure of 3Cr/9+14 as shown in

FIG. 1 is used but the roundnesses b/a and b′/a′ are too small, andComparative Example 8 is the case of using the steel cord of thelayer-twisted structure of 3Cr/9+13 as shown in FIG. 8.

As is clear from Table 1 and Table 2, it can be seen that Examples 1 to6 of the invention are excellent in each of the rubber permeability andthe fatigue resistance as compared with Related art examples 1 and 2 andComparative Examples 1 to 8.

As described above, because the rubber reinforcing steel cord of thepresent invention is the steel cord of a bundle-twisted+layer-twistedstructure obtained by arranging 9 steel filaments around 3 steelfilaments each being two-dimensionally wave-like shaped in thelengthwise direction, intertwining all of these steel filaments in oneoperation to form a bundle-twisted cord, and further arranging 14 steelfilaments around the bundle-twisted cord followed by intertwining,wherein when the diameter of the two-dimensionally wave-like shapedsteel filament is d, the wave-form height thereof is h, and the waveform pitch thereof is P, the parameter F shown by (h−d)/P is in therelation of 0.001≦F≦0.03 and further, the ratio b/a of the long diametera and the short diameter b of the circumscribed circle of the crosssection of the bundle-twisted cord is 0.94≦b/a≦1.00, and the ratio b′/a′of the long diameter a′ and the short diameter b′ of the circumscribedcircle of the cross section of the steel cord is 0.96≦b′/a′≦1.00, itbecomes possible to increase the rubber permeability, the fatigueresistance, and the productivity.

Also, the rubber reinforcing steel cord of the invention has theadvantages such as (a) because the three steel filaments of the coreportion are not simply arranged but are intertwined together with ninesteel filaments of the sheath just outside of the core portion, when atension is applied to the steel cord, the tension burden is notconcentrated (unbiased) to the core portion, (b) by reducing the numberof the steel filaments of the outermost layer to the minimum necessarynumber, the diameters of the steel filaments are not thickened (thefatigue resistance of the steel cord is good), (c) because the rubberpermeability is good, a rap filament can be omitted, etc.

Furthermore, by using the rubber reinforcing steel cord as the cords ofthe carcass layers of a heavy duty pneumatic radial tire, the durabilityof the heavy duty pneumatic radial tire can be increased.

What is claimed is:
 1. A rubber reinforcing steel cord comprising asteel cord of a bundle-twisted+layer-twisted structure obtained byarranging 9 steel filaments around 3 steel filaments, the 3 steelfilaments each being two-dimensionally waved in the lengthwisedirection, intertwining all of these steel filaments in one operation toform a bundle-twisted cord, and further arranging 14 steel filamentsaround the bundle-twisted cord followed by intertwining, wherein whenthe diameter of the two-dimensionally waved steel filament is d, thewave-form height thereof is h, and the wave form pitch thereof is P, theparameter F, defined by (h−d)/P is in the relation of 0.001≦F≦0.03 andfurther, the ratio b/a of the long diameter a and the short diameter bof the circumscribed circle of the cross section of the bundle-twistedcord is 0.94≦b/a≦1.00, and the ratio b′/a′ of the long diameter a′ andthe short diameter b′ of the circumscribed circle of the cross sectionof the steel cord is 0.96≦b′/a′≦1.00.
 2. The rubber reinforcing steelcord according to claim 1 wherein the twisting pitch P₁ of thebundle-twisted cord and the twisting pitch P₂ of the 14 filamentsarranged around the bundle-twisted cord are in the relation ofP₂≧2.5+P₁.
 3. A heavy duty pneumatic radial tire comprising a carcasslayer of rubber reinforcing steel cords having abundle-twisted+layer-twisted structure obtained by arranging 9 steelfilaments around 3 steel filaments, the 3 steel filaments each beingtwo-dimensionally waved in the lengthwise direction, intertwining all ofthese steel filaments in one operation to form a bundle-twisted cord,and further arranging 14 steel filaments around the bundle-twisted cordfollowed by intertwining, wherein when the diameter of thetwo-dimensionally waved steel filament is d, the wave-form heightthereof is h, and the wave form pitch thereof is P, the parameter Fdefined by (h−d)/P is in the relation of 0.001≦F≦0.03 and further, theratio b/a of the long diameter a and the short diameter b of thecircumscribed circle of the cross section of the bundle-twisted cord is0.94≦b/a≦1.00, and the ratio b′/a′ of the long diameter a′ and the shortdiameter b′ of the circumscribed circle of the cross section of thesteel cord is 0.96≦b′/a′≦1.00.
 4. The heavy duty pneumatic radial tireaccording to claim 3 wherein the twisting pitch P₁ of the bundle-twisted cord and the twisting pitch P₂ of the 14 filaments arrangedaround the bundle-twisted cord are in the relation of P₂≦2.5+P₁.